Volatile management during a curing cycle becomes a critical issue for epoxy matrix-based composites.
The traditional Single Vacuum Bag (SVB) process without additional pressure generated by an autoclave normally fails to yield void-free quality composites for epoxy matrix-based composites because of the volatiles (solvents and reaction by products).
A variation of this SVB system uses two vacuum bags during a one step of the cure cycle. This double vacuum bag debulking (DVD) process for volatile and trapped air management in composite materials performs better than the traditional Single Vacuum Bag (SVB) process.
The definition of the double vacuum bag is a vacuum bag process using atmospheric pressure alone that eliminates the need for external pressure supplied normally by an autoclave for composite fabrication.
Double Vacuum Bag Debulking processes are known techniques already described in different documents such as U.S. Pat. No. 6,761,783, which describes a method for repairing composite aircraft structures. FIG. 1 of this application corresponds to FIG. 6 of U.S. Pat. No. 6,761,783. As shown in FIG. 1, a method to repair bismaleimide (BMI) composite structures which includes the steps of consolidating the repair patch by vacuum bagging the repair patch and additional layers with a lower vacuum bag and placing a fiberglass cloth over the lower vacuum bagging, a rigid box over the fiberglass cloth and an additional fiberglass over the rigid box and vacuum bagging with an upper vacuum bag joined to the lower vacuum bagging with sealant. In the first step a first vacuum is applied to the lower vacuum bag and then a second vacuum is applied to the upper bag so that the second vacuum is at a level ranging between approximately zero inches to one inches of mercury less than the first vacuum such that the upper vacuum level is equal to or slightly less than the lower vacuum level (tolerance is +0, −1 inches of mercury). See U.S. Pat. No. 6,761,783, col. 11, Ins. 6 to 9. In this case, the outer bag is collapsed onto the stiff perforated tool with a pressure (less than one atmospheric pressure). Because of the vacuum differential between the two bags, the inner bag is collapsed, instead of “ballooned”, and presses against the composite with a small compaction pressure, which might hinder to some degree an efficient and complete removal of trapped air and volatiles. “If the vacuum in the upper chamber were greater than the vacuum in the lower vacuum bag, the vacuum in the upper chamber would effectively suck the lower vacuum bag and materials up into the upper chamber, damaging or destroying the repair material.” U.S. Pat. No. 6,761,783, col. 11, Ins. 9 to 13. To prevent this issue the upper vacuum bag should be sealed 100% beyond the sealing points of the inner vacuum bag, with the rigid box between the two sealing points, so that the higher vacuum pressure in the outer bag presses the sealing points of the inner bag and keeps them properly in place avoiding their detachment by the higher pressure in the outer vacuum bag. In the system depicted in this document, this is not possible because of the location of the vacuum source in the inner vacuum bag (item 610 in FIG. 1).
U.S. Pat. No. 5,236,646 relates to a manufacturing process of substantially void-free consolidated thermoplastic composite, by two independent low-pressure vacuum chambers comprising a vacuum bag that creates an inner chamber that totally covers the layers or plies of thermoplastic material and an “outer-rigid vacuum chamber, consisting of steel etc., open-ended box, is set over the entire lay-up so that the underside edges of the box, having additional sections of sealing tape secured thereto, fit securely and firmly onto the upper surface of the flexible bag thereby ensuring that a hermetically sealed second vacuum chamber is formed between the outer surface of the flexible bag, i.e. inner chamber, and the rigid chamber and allowing the peripheral area of bag to extend therebeyond. An aperture connects into the outer chamber thereby formed to allow connection of a second vacuum, as shown by gauge. “U.S. Pat. No. 5,236,646, col. 5, In. 65 to col. 6, In. 8 (reference numbers removed from quotation). The process consists on simultaneously applying a vacuum in the first chamber and a second vacuum under the rigid vacuum chamber. During the initial step, the vacuum in the said outer chamber may range from about 0 to 2 inches of Hg, more or less, i.e. plus or minus, than the vacuum in said inner chamber until substantially all the volatiles are removed. U.S. Pat. No. 5,236,646, col. 7, Ins. 9-16. When the vacuum in the outer chamber is 2 inches of Hg less than in the inner vacuum bag, this will collapse and as discussed above the volatile extraction will be hindered to a certain degree. When the vacuum in the outer chamber is 2 inches of Hg more than in the inner vacuum bag, this will balloon and will remain stable in place if the underside edges of the rigid chamber are sealed and firmly secured onto the outer surface of the inner bag. In the manufacture of three dimensional components (sandwich panels with chamfered areas, omega or T stringers, etc.) the proper and complete sealing of the outer rigid vacuum chamber could be hardly achieved if the inner vacuum bag has been folded in the relevant zones to accommodate and adapt to the component shape. Additionally, this system cannot be applied for repairing processes because the vacuum source in the inner vacuum bag is located in the manufacturing tool inside the area covered by the outer rigid chamber (item 73 in FIG. 2 which is taken from FIG. 3 of U.S. Pat. No. 5,236,646).
In addition to these documents reviewed above, publication Hou et al, “Evaluation of Double-Vacuum-Bag Process for composite Fabrication” (NASA Langley Research Center, Hampton, Va. 23681) describes another composite manufacturing process of void-free high quality laminate based on double vacuum bagging technique (FIG. 3 of this application corresponds to FIG. 3 of the Hou NASA publication 2004).
The Hou NASA publication described a fiber reinforced reactive resin matrix prepregs that are laid up between the caul and the tool steel plates. They are then enclosed by a vacuum bag (designated as Inner Bag), which is sealed around the edges onto the tool plate. A vacuum port is built on the tool plate inside the Inner Bag and connected to a vacuum pump as with the SVB process. A second vacuum bag (designated as Outer Bag) is then assembled in the same fashion, with a vacuum port built on the tool plate, which is located between the inner and outer bags and connected to a separate vacuum pump. Before assembling the outer bag, a perforated tool is first installed outside the perimeter of the Inner Bag. This tool has to be stiff enough to withstand the 14.7 Psi atmospheric pressure created by the vacuum.
During the B-stage (i.e., the low temperature ramp-and-hold period), full vacuum (30″ Hg) is applied to the Outer Bag, while a slightly lower vacuum level (i.e., 28″ Hg) is set in the Inner Bag. The Outer Bag is collapsed onto the stiff perforated tool due to the atmospheric pressure outside the bags. Because of the vacuum differential between the two bags, the Inner Bag is “ballooned” and presses against the perforated stiff tool leaving no compaction force, while still producing vacuum, on the composite. In the DVB arrangement, the composite lay-up assembly is not compacted by the atmospheric pressure via the Inner Bag, and remains loose. Volatiles are free to escape by the vacuum suction from the Inner Bag vacuum pump during the B-stage.
“At the end of the B-stage, the Outer Bag is purged to atmosphere, while the Inner Bag vacuum is increased to 30″ Hg. The Outer Bag becomes loose from the tool, and the Inner Bag collapses onto the caul plate with one atmospheric pressure. This pressure helps to consolidate the laminate during the high temperature ramp-and-hold period of the cure cycle.” Hou NASA publication, page 6.
The Hou NASA publication also discusses the possibility of using the system in the opposite way, by applying a lower vacuum pressure to the outer chamber than to the inner bag, so that the inner bag is collapsed and presses against the composite with a small compaction pressure. It is argued that the volatile depletion will not be hindered by the slightly compacted fibrous architecture and it is mentioned that the potential for inner bag leakage is greatly reduced. This risk had been already described in U.S. Pat. No. 6,761,783 where this way of working (lower pressure in the outer bag than in the inner bag) was also chosen. Even if this system has a different arrangement for the sealing lines and the vacuum sources, this system also presents the same issue described above for the system of document U.S. Pat. No. 6,761,783, that is: the inner vacuum bag when it is “ballooned” might drag the seal tape and break the sealing lines. The extent to which the air trapped between plies during the lay-up and the volatiles are effectively removed with a small compaction pressure acting on the inner bag will depend on the type and on the thickness of the fiber reinforcement. The thicker and the tighter the reinforcement, the more difficult the trapped air and volatiles removal against the compaction pressure will be.
This system is not applicable to repairs because of the positions of the vacuum sources in the tool.